Replacement of diseased or injured joints is common practice in orthopedic surgery. The replacement of a shoulder joint either for arthritic disease or for fracture by an artificial device can restore function in many cases.
Shoulder prostheses can be implanted in the presence of the so-called “four part humeral fractures”. In those fractures of the proximal part of the humerus, the humeral head, the greater tuberosity and the lesser tuberosity separate from the humeral shaft. This results in the four parts fractures described by Neer. In such cases, it is common practice to use shoulder prosthesis to reconstruct the proximal part of the humerus. However, if replacing the humeral head is relatively easy, reattaching the tuberosities often results in failure of those parts to unite to the rest of the humerus or in their union in an inappropriate position. Bad functional outcome of shoulder replacement for fracture seems to be directly correlated to this phenomenon. A number of prosthesis have been described and used to solve the problem. Among others, see for instance U.S. Pat. No. 6,171,341 and No. 5,944,758, No. 6,283,999. In those devices there are various means to keep the tuberosities in position until union. The U.S. Pat. No. 6,334,874 B1, like U.S. Pat. No. 3,228,393 for a femur prosthesis, has a metaphyseal portion with window in which a bone graft can be inserted and U.S. Pat. No. 6,171,341 has a metaphyseal portion comprising only a medial branch thereby having an area in which a bone graft can be used. In those two devices a better contact between the fractured fragments is sought. However, in the prior art, the stability of the fragments necessary to fracture healing and the provision of abundant biological material necessary to induce the union of the bony fragments are not simultaneously provided.
In another aspect of shoulder prosthesis designing relevant to the present invention, U.S. Pat. No. 6,368,352, in order to diminish the weight of the implant, has a closed space defined between the collar and the modular head in an assembled configuration while DE patent No 4220217 has a head which is a cap with a central shaft and an inner surface with open cell or a porous structure.
In the present invention, the space of the hollow head is completely open in order to accommodate bone grafts.
The object of the invention is to restore shoulder function after three or four parts humeral fractures by providing an improved prosthesis to bring about the union of the separated tuberosities to the upper humerus while replacing the broken humeral head. To do so a modular composite shoulder prosthesis is described whose construct incorporates a solid autogenous bone graft. The prosthesis comprises a stem to be inserted in the medullary canal of the humerus, an intermediary part with a number of holes and a head that is hollow. The head of the prosthesis being a hollow spherical cap with an inner lining of an osteoconductive material, that is, a material conductive to bone ingroth the lateral aspect of the medial pillar as well as the upper part of the stem being also coated with the same material, a large epiphyso-metaphyseal space is created in which a solid autogenous bone graft usually from the iliac crest can be impacted. Union can be achieved between the prosthesis and the autogenous iliac bone graft on one side and between the iliac bone graft, the tuberosities reattached to the humerus shaft, to the intermediary portion and between themselves by multiple sutures and/or wires. The bone graft is considered as a part of the prosthesis both from a mechanical and from a biological perspective.
This composite shoulder prosthesis results in the following:                1. a structure formed by the cap, the ring, the medial pillar and the stem, that provides a smooth spherical surface together with mechanical support and stability;        2. an osteoconductive coating that provides a framework amenable to the ingrowth of blood vessels and osteoprogenitor, type of bone cell consisting of undifferentiated cells from which the osteoblasts, the bone-forming cells, are derived;        3. osteoinductive factors brought by the iliac graft for the induction or the modulation of bone formation;        4. osteogenic cells, graft cells from the iliac bone graft that can differentiate into osteoblasts.        
Accordingly, the mechanical and biological conditions to facilitate the union process between those humeral tuberosities, the composite prosthesis, and the remainder of the humerus are optimally set.
In certain cases in which the harvesting of the iliac bone graft is considered of an excessive morbidity, the prosthesis is also provided with a scaffold of an osteoconductive material, scaffold in which bone chips from the broken humeral head can be positioned for bone induction.